Importacular allows you to avoid adding duplicate data into the Raiser's Edge in two different ways.
1) By working with user defined Criteria Sets, you can select how to match the constituents in your file to constituents that already exist in the Raiser's Edge. You may also find this knowledgebase containing common recommendations helpful. **If you are using the same data source to import more than once, you should always close the review screen after your first import and allow the Criteria Sets to match the data to the Raiser's Edge a second time so that you don't accidentally create the same records more than once.**
2) In addition to avoiding duplicate Constituent entry, you can also work with the Area Settings for most area mappings in your template to avoid adding duplicate data to the Constituent's record. One example of this can be seen in this knowledgebase article regarding avoiding duplicate phone numbers. Similar functionality is available on the Area Settings for other fields (including but not limited to: Attributes, Addresses, Gifts, Ratings, Individual Relationships, etc.).